1. Field of the Invention
The present invention is broadly concerned with a method and apparatus for the selectively controlled switching or gating of superconducting shields used in conjunction with magnetometers or other field detectors in order to permit selective detection of the location of the source, frequency and/or amplitude of a desired field (either magnetic, electric or electromagnetic). More particularly, it is concerned with controllable shields in the form of multiply-connected bodies of superconducting material wherein selective switching is accomplished by localized lowering of the critical shielding current density of a portion of a shield body, while maintaining the critical shielding current densities of other portions of the bodies at higher, field-shielding levels; preferably, localized critical shielding current density lowering is accomplished by application of direct current adjacent the shields, which induces corresponding local bias magnetic fields serving to suppress the current density. Shields in accordance with the invention find application in biomedical magnetic detectors, SQUID arrays used for detection of magnetic anomalies, and for direction-specific reception of electromagnetic signals.
2. Description of the Prior Art
Superconducting materials exhibit unique physical properties which are useful for effectively shielding (i.e., preventing penetration of) external magnetic, electric and electromagnetic fields. The characteristic shielding property of superconductors arises as a function of an inherent but measurable parameter of the superconductor, which can be termed the "critical shielding current density." In order to understand the shielding phenomenon, consider that when a superconducting material is sufficiently cooled below its critical temperature (T.sub.c), it is essentially diamagnetic. Therefore an external field acting on the cooled superconductor creates within the structure of the superconductor an induced current, and a corresponding counter or canceling field which prevents passage of the external field through the superconductor. As the magnitude of the external field acting on the superconductor is increased, the induced current level within the superconductor also increases to generate an increasing counter field. At the same time, the critical shielding current density concomitantly decreases, until the latter is reduced essentially to zero. At this point, the superconductor can no longer generate additional induced current and counter field, and the external field will penetrate the superconductor shield. The existence and extent of the shielding phenomenon is therefore dependent upon the critical shielding current density of the superconductor employed.
The critical shielding current density and thus the overall shielding capability of a given superconductor under varying ambient conditions depends on a number of factors: (1) the critical temperature (T.sub.c) of the superconductor and its dependence on external magnetic fields; (2) the distribution of magnetic field lines inside the superconductor; (3) the intragrain and intergrain critical current densities (J.sub.c) and their dependence on external magnetic fields and temperature; (4) the pinning potential for magnetic fluxoids and the amount of trapped fluxoids in the superconductor; and (5) the connectivity of superconducting grains and the capability of carrying a macroscopic closed-loop supercurrent in the bulk material to screen out and shield external fields.
It has been suggested in the past to employ superconducting shields in various applications, and particularly in conjunction with high sensitivity magnetometers such as SQUIDS (superconducting quantum interference devices). In general, these prior devices include a SQUID assembly made up of a SQUID and an associated flux transformer having a superconductive shield disposed about the SQUID assembly but leaving an open, non-shielded detection window oriented for passage of the desired field towards the central detector. The goal is therefore to shield the SQUID assembly from undesired ambient fields, while permitting entry of the desired field through the detection opening. Such magnetometers have heretofore found application in non-invasive biomedical devices designed to detect the low magnitude magnetic fields emanating from body organs such as the heart, skeletal muscles, brain, lungs and eyes. They may also be used for airborne geological surveys, or in arrays for the passive detection of magnetic anomalies. A persistent problem with such devices stems from the fact that they are very sensitive, and therefore have difficulty in discriminating desired fields arising from the phenomenon under investigation, and undesired background fields. Thus, the signal-to-noise ratio of these detectors is relatively small. By the same token, these units have difficulty in determining the precise location, frequency and magnitude of a field source.